# Flow velocity

In continuum mechanics the flow velocity in fluid dynamics, also macroscopic velocity[1][2] in statistical mechanics, or drift velocity in electromagnetism, is a vector field used to mathematically describe the motion of a continuum. The length of the flow velocity vector is the flow speed and is a scalar. It is also called velocity field; when evaluated along a line, it is called a velocity profile (as in, e.g., law of the wall).

## Definition

The flow velocity u of a fluid is a vector field

${\displaystyle \mathbf {u} =\mathbf {u} (\mathbf {x} ,t),}$

which gives the velocity of an element of fluid at a position ${\displaystyle \mathbf {x} \,}$  and time ${\displaystyle t.\,}$

The flow speed q is the length of the flow velocity vector[3]

${\displaystyle q=\|\mathbf {u} \|}$

and is a scalar field.

## Uses

The flow velocity of a fluid effectively describes everything about the motion of a fluid. Many physical properties of a fluid can be expressed mathematically in terms of the flow velocity. Some common examples follow:

### Steady flow

The flow of a fluid is said to be steady if ${\displaystyle \mathbf {u} }$  does not vary with time. That is if

${\displaystyle {\frac {\partial \mathbf {u} }{\partial t}}=0.}$

### Incompressible flow

If a fluid is incompressible the divergence of ${\displaystyle \mathbf {u} }$  is zero:

${\displaystyle \nabla \cdot \mathbf {u} =0.}$

That is, if ${\displaystyle \mathbf {u} }$  is a solenoidal vector field.

### Irrotational flow

A flow is irrotational if the curl of ${\displaystyle \mathbf {u} }$  is zero:

${\displaystyle \nabla \times \mathbf {u} =0.}$

That is, if ${\displaystyle \mathbf {u} }$  is an irrotational vector field.

A flow in a simply-connected domain which is irrotational can be described as a potential flow, through the use of a velocity potential ${\displaystyle \Phi ,}$  with ${\displaystyle \mathbf {u} =\nabla \Phi .}$  If the flow is both irrotational and incompressible, the Laplacian of the velocity potential must be zero: ${\displaystyle \Delta \Phi =0.}$

### Vorticity

The vorticity, ${\displaystyle \omega }$ , of a flow can be defined in terms of its flow velocity by

${\displaystyle \omega =\nabla \times \mathbf {u} .}$

Thus in irrotational flow the vorticity is zero.

## The velocity potential

If an irrotational flow occupies a simply-connected fluid region then there exists a scalar field ${\displaystyle \phi }$  such that

${\displaystyle \mathbf {u} =\nabla \mathbf {\phi } .}$

The scalar field ${\displaystyle \phi }$  is called the velocity potential for the flow. (See Irrotational vector field.)

## Bulk velocity

In many engineering applications the local flow velocity ${\displaystyle \mathbf {u} }$  vector field is not known in every point and the only accessible velocity is the bulk velocity (or average flow velocity) ${\displaystyle U}$  which is the ratio between the volume flow rate ${\displaystyle {\dot {V}}}$  and the cross sectional area ${\displaystyle A}$ , given by

${\displaystyle u_{\rm {{}av}}={\frac {\dot {V}}{A}}}$

where ${\displaystyle A}$  is the cross sectional area.

## References

1. ^ Duderstadt, James J.; Martin, William R. (1979). "Chapter 4:The derivation of continuum description from transport equations". In Wiley-Interscience Publications (ed.). Transport theory. New York. p. 218. ISBN 978-0471044925.
2. ^ Freidberg, Jeffrey P. (2008). "Chapter 10:A self-consistent two-fluid model". In Cambridge University Press (ed.). Plasma Physics and Fusion Energy (1 ed.). Cambridge. p. 225. ISBN 978-0521733175.
3. ^ Courant, R.; Friedrichs, K.O. (1999) [unabridged republication of the original edition of 1948]. Supersonic Flow and Shock Waves. Applied mathematical sciences (5th ed.). Springer-Verlag New York Inc. pp. 24. ISBN 0387902325. OCLC 44071435.